1. Field of Invention
This invention relates generally to the field of reservoir engineering and analysis. In particular the invention concerns creating a numerical model or simulation of a subterranean oil and gas reservoir. Still more particularly, the invention is for a method and apparatus for creating an areal grid within a reservoir boundary area for creating reservoir cells the shapes of which are defined in the areal grid projected vertically downwardly into the earth and by layering boundaries.
2. Description of Prior Art
A reservoir model is used by petroleum engineers to predict reservoir performance by numerically solving flow equations which describe fluid flow in petroleum reservoirs during the oil and gas recovery process. By using the model, various reservoir recovery plans can be simulated in order to find the one which best meets the reservoir recovery objectives.
In the petroleum industry, reservoir simulation has been primarily conducted by dedicated specialists. There is a trend within the industry to make reservoir simulation a tool for day-to-day use by members of asset teams. However, prior art simulators are too complex for the non-expert users.
To create a model of the reservoir, simulation engineers first divide an areal surface into grid cells which are used to define grid blocks by layers of the grid cells extending into the reservoir structure. A numerical model is created by choosing a grid orientation, number and distribution of grid lines. Then wells are assigned to grid cells based on their locations. There can be millions of grid cells and hundreds of production and injection wells in a simulation model. For any change with the grid, well number and well locations, engineers must make changes to the cells to account for changes in the number and location of the wells, etc.: a very tedious process.
Besides the difficulties in setting up a numerical reservoir model, a simulation engineer also is concerned with the quality of the simulation grid and model. A desired grid should provide high numerical resolution in the reservoir regions where the simulated reservoir rock and fluid properties change rapidly with time and space. To this end, great efforts have been made in developing the local grid refinement technology in the past decades. One method has included replacing the coarse Cartesian grid with a fine Cartesian, radial or PEBI grid around the wellbores. (A PEBI grid is a Perpendicular Bisection grid, also known as a Voronoi grid of Voronoi blocks. A Voronoi block is defined as the region of space that is closer to its gridpoint than any other gridpoint. A consequence of this is that a line joining gridpoints of any two connected gridpoints is perpendicular to the gridblock boundary between these two gridpoints and its is bisected in two equal parts by that boundary.) Generally speaking, a radial grid defined model performs better for local grid refinement than a PEBI and Cartesian grid for local grid refinement. The radial grid has long been used for near wellbore precision modeling and single well reservoir modeling, because it can better describe the fluid flow process in reservoirs. However, no such radial gridding methods are known for multiple well simulation problems.
Another consideration for reservoir simulation grids is the reservoir boundary and faults conformance problem. Almost all the reservoirs are irregularly shaped and may have one or more faults. A prior solution involves removing coarse Cartesian grid cells in the concerned regions, replacing them with fine structured or non-structured grid cells, and smoothing out the transition from frame Cartesian cells to these special grid cells. Such a process introduces a great many extra grid cells. As far as the simulation accuracy concerns, it is only necessary to know the locations of the reservoir boundary and irregularities (including faults and pinch-outs). There is no dramatic physical properties change happening at such locations. In other words fine or special grid cells are not necessary for describing the reservoir shape.